The present invention relates to an ignition coil for an internal-combustion engine and, more particularly, to an ignition coil for an internal-combustion engine, capable of being partly or entirely set in a plug hole formed in the internal-combustion engine.
The sectional area of a center core included in an ignition coil for an internal-combustion engine, capable of being set in a plug hole of the internal-combustion engine is limited by the diameter of the plug hole. The magnitude of the magnetic energy, i.e., the discharge energy, of the ignition coil is substantially proportional to the sectional area of the core. Ignition coils proposed in, for example, Japanese Patent Laid-open Nos. Hei 4-87311 and 9-167709 employ a center core having a substantially circular cross section. An ignition coil proposed in, for example, Japanese Utility Model Laid-open No. Hei 1-120314 (U.S. Pat. No. 4,893,105) employs a center core tapering from the side of a primary coil toward the side of a secondary coil.
Since the diameter of, the plug hole places a restriction on the diameter of the prior art ignition coil, it is difficult to form the insulators in a sufficient diametrical size and hence there is a limit to the enhancement of the durability of the ignition coil through the improvement of insulation capacity.
Accordingly, it is an object of the present invention to provide an ignition coil for an internal-combustion engine, capable of generating a sufficient amount of discharge energy and having improved electrical insulation capacity, and to provide an internal-combustion engine provided with such an ignition coil.
Since there is a limit to the diametrical size of an ignition coil to be set in a plug hole of an internal-combustion engine, the ignition coil is provided separately with a center core and a side core to facilitate insulation. Accordingly, the ignition coil has an open magnetic circuit having a big gap length between the opposite ends of a center core. It was found through the studies of the magnetic characteristics of an ignition coil of an open magnetic circuit type that leakage magnetic flux increases in the vicinities of the opposite ends of the center core, magnetic flux is distributed on the center core so that magnetic flux density decreases from the middle of the center core toward the opposite ends of the center core, and magnetic flux density at the opposite ends of the center core is about ⅓ of that at the middle of the center core.
The present invention is characterized by a center core formed so that the sectional area of sections of portions of the center core on the inner side of the opposite ends of the center core perpendicular to the axis of a coil axis is the greatest. The center core having such a shape prevents the reduction of the magnetic characteristic (magnetic energy) thereof.
The present invention is characterized by a center core having a stepped longitudinal section having a stepped width decreasing stepwise from the middle portion toward the opposite ends of the stepped longitudinal section. Magnetic flux density distribution on the center core having such a stepped longitudinal section is substantially uniform and hence discharge energy, i.e., magnetic energy, is not reduced.
Since the center core having such a stepped longitudinal section provides sufficient spaces in the vicinities of the opposite ends thereof where potential difference is large, the center core and a secondary coil can satisfactorily be insulated from each other, which improves reliability in electrical insulation and durability. Since end portions of the center core have a stepped shape, induction of eddy current can be suppressed, core loss can be reduced and rise in the temperature of the ignition coil can be suppressed.
To prevent the reduction of the magnetic characteristics (magnetic energy) of the center core, it is desirable that the cross-sectional area of the center core reach a maximum in portions on the inner side of the opposite ends of the center core. As shown in FIG. 7 by way of example, if an allowable magnetic energy reduction ratio is 10%, it is desirable that the cross-sectional area of the center core reach a maximum in portions at distances in the range of 20% of the length of the center core from the middle of the center core. It is desirable that the middle portion of the center core have the greatest cross-sectional area to prevent the reduction of the magnetic characteristics (magnetic energy) of the center core.
The magnitude of the magnetic energy of the center core is substantially proportional to the sectional area of the center core. As indicated by a curve for a comparative example in FIG. 7, in an open magnetic circuit having a gap between a center core and a side core as this ignition coil, the effect of the sectional area of the center core is less utilized in portions nearer to the opposite ends of the center core. As indicated by a curve for a first embodiment in FIG. 7, magnetic energy does not decrease even if the sectional area of the core is reduced toward the opposite ends of the center core.
The above and other features of the present invention will be described hereinafter with reference to the accompanying drawings.